Retaining walls are used in various landscaping projects and are available in a wide variety of styles. Numerous methods and materials exist for the construction of retaining walls. Such methods include the use of natural stone, poured concrete, precast panels, masonry, and landscape timbers or railroad ties.
A widely accepted method of construction of such walls is to dry stack concrete wall units, or blocks. These blocks are popular because they are mass produced and, consequently, relatively inexpensive. They are structurally sound, easy and relatively inexpensive to install. Because they comprise concrete, they are durable. They can be given a desired appearance, such as, for example, natural stone. Many block systems also use pins that are adapted to fit in corresponding pin holes in adjacent blocks or may use other mechanical means to contribute to the stability of a wall.
Typically, retaining wall blocks are manufactured to have the desired appearance on the front face (i.e., the outer face of a wall) because only the front is visible after the wall is constructed. It is highly desirable to have the front face of the wall system have a natural stone appearance, and many approaches are used in the art to treat or process concrete to evoke the appearance of natural stone, including splitting the block, tumbling the block to weather the face and edges of the face, and using processing or texturing equipment to impart a weathered look to the concrete.
Depending upon their location, the soil type, the amount of water that can flow through the wall, and the mineral content of the water, an undesirable appearance can develop on the surface of a retaining wall. In addition, due to exposure to the elements and freeze/thaw cycles, concrete retaining walls may exhibit spalling, that is, chipping and cracking of concrete, which affects their appearance and can ultimately affect their utility. Freeze-thaw effects are worsened when the wall face is exposed to salt spray, which commonly occurs on roadways where de-icing salts are used to clear the road of ice and snow. Efflorescence refers to the leaching of mineral salts from water and this often occurs on walls in contact with water. The resultant deposit on a surface creates an unattractive white stained appearance on a wall.
There have been prior efforts to veneer segmental retaining walls with natural stone or concrete that is molded to closely resemble natural stone. While such veneering produces aesthetically pleasing walls, it is a laborious and highly expensive process, as it requires skilled masonry work to tie in the stone or concrete veneer to the wall using traditional mortared masonry construction methods. Such veneering can double the cost of the finished wall. In addition, segmental retaining walls are not rigid structures and applying a rigid mortared veneer may cause cracking unless appropriate steps are taken to provide slip joints.
Accordingly, it would be desirable to provide a retaining wall system that would be easy to install, that would possess an appearance that closely resembles natural stone and that would keep its desirable appearance indefinitely. Another need in the art is a way to improve the appearance of surface-damaged or stained retaining walls.
Many retaining wall systems described in the art include the use of reinforcing materials, also referred to as geogrids, geosynthetic reinforcement, or geogrid soil reinforcement. These terms sometimes are used interchangeably, and “geogrid” as used herein is intended as a generic term. Reinforcement materials may be inextensible, such as steel mesh, or extensible geosynthetic materials, such as mats and oriented polymeric materials. For example, flat polymeric sheets are used to form geogrids by forming holes in the sheets and then drawing them to orient the polymer and increase the modulus. Such polymeric materials include high density polyethylene (HDPE) and these materials form relatively stiff geogrids commercially available under the trade designation “TENSAR”.
While the HDPE materials are relatively stiff, a second type of geosynthetic material is composed of a mat typically formed from polyester fibers that are woven or knitted. These may comprise rectilinear polymer constructions characterized by large (e.g., 1 inch (2.5 cm) or greater) openings. In these open structure geogrids, polymeric strands are woven, knitted or “welded” (by means of adhesives and/or heat) together in a grid. Polymers used for making relatively flexible geogrids include polyester fibers. The polyester typically is coated, commonly using a polyvinyl chloride (PVC) or a latex topcoat. The coating may contain carbon black for ultraviolet (UV) stabilization. Some open structure geogrids comprise polyester yarn for the warp fibers and polypropylene as the fill fibers.
Another flexible reinforcing geosynthetic material is fabric, i.e., woven or non-woven constructions without large openings. These fabrics typically comprise polymers and may be referred to as geofabrics. The geofabric can be laid between courses of blocks in a wall, and typically is tied into the wall and held there. When blocks are configured to have pin connectors, for example, a hole or slit is formed in the geofabric at the construction site and the geofabric is held on the blocks by fitting it over the pins.
In common use, the geogrid extends behind the retaining wall and ties into the earth behind the wall, thus creating a cohesive soil mass tied into the wall facing that resists overturning. Geogrids are either mechanically connected to a course of blocks or rely on the friction created by placing the geogrids layer between courses of blocks. When the mode of connection is friction alone, the geogrid is placed on top of a course of blocks, and then a succeeding layer of blocks is placed on top of the geogrid. When the connection is mechanical, after placement of a course of blocks to the desired height, geogrid is placed onto a course of blocks and held in place by means of pins in the block (which may have a primary function of aligning and holding blocks together) or by means of special connectors. Flexible geogrid is put under tension by pulling back and staking the geogrid behind the retaining wall. Backfill is placed and compacted over the geogrid. Construction of the wall continues and may include additional layers of geogrid.
Such systems have proven reliable in many wall applications. There are limits to their performance however, particularly at the upper portions of the wall, where the load of the blocks above the geogrids layer do not provide as much load on the connection, so that frictional forces are reduced. The use of frictional connections forces the wall designer and builder to use more and higher strength geogrids because connection strength limits the strength of the system, and this adds expense to the wall. Mechanical connectors attempt to overcome this limitation by mechanically connecting the geogrids to the wall facing in a way that is not load dependent. The difficulty with this approach is that in order for such connectors to provide high levels of efficient connection they must add considerable expense to cost of using the geogrids reinforcement, and add complexity and expense to the installation process. Thus there are shortcomings to both approaches.
There are also many wall applications in seismic zones where providing an extra measure of protection against loss of connection between the wall facing and the reinforcing geogrids during a seismic event would be highly desirable. Thus there is a need for a geogrid connection system that is not load dependent, that does not add significant expense to the construction of the wall, and that is highly reliable and resistant to failure during seismic conditions.